Hitherto, a rubber composition comprising a polybutadiene containing a cis-1,4 bond of not less than 80%, synthesized using nickel-containing catalyst, has been suitably used as a rubber composition for a one-piece solid golf ball, a core of a two-piece golf ball or three-piece golf ball, or a solid center of a thread wound golf ball, when considering rebound characteristics and durability. It is also known that a polybutadiene, synthesized using lanthanide-containing catalyst, can be used for the same application.
It is disclosed, for example, in Japanese Patent Kokoku Publication No. 80123/1994 that a mixture consisting of                (i) a polybutadiene containing a cis-1,4 bond of not less than 40%, having a Mooney viscosity of 70 to 100 ML1+4(100° C.), synthesized using nickel-containing catalyst and/or cobalt-containing catalyst, and        (ii) a polybutadiene containing a cis-1,4 bond of not less than 40%, having a Mooney viscosity of 30 to 90 ML1+4 (100° C.), synthesized using a catalyst consisting of lanthanide rare earth elements-containing compound, or a polybutadiene containing a cis-1,4 bond of not less than 40%, having a Mooney viscosity of 20 to 50 ML1+4(100° C.), synthesized using nickel-containing catalyst and/or cobalt-containing catalyst can be suitably used as a rubber composition for golf balls. When polybutadiene (i) is used for the rubber composition, the rebound characteristics of the resulting golf ball are improved. However, when polybutadiene (ii) having a high Mooney viscosity is used in combination with polybutadiene (i), the workability in processing is degraded.        
It is disclosed in Japanese Patent No. 2644226 that a polybutadiene containing a cis-1,4 bond of not less than 80% and having a Mooney viscosity of 45 to 90 ML1+4(100° C.) and a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 4.0 to 8.0 can be suitably used for golf balls. Although the use of the polybutadiene imparts sufficient physical properties to golf balls, improvement in rebound characteristics and durability is further required.
It is disclosed in Japanese Patent No. 2678240 that a mixture of                85 to 15 phr of a nickel- or cobalt-catalyzed polybutadiene having a cis-1,4 content of more than 40% and a Mooney viscosity of less than 50, and        15 to 85 phr of a lanthanide-catalyzed polybutadiene having a cis-1,4 content of more than 40% and a Mooney viscosity of less than 50 can be suitably used for a golf ball. However, when using a mixture of the nickel- or cobalt-catalyzed polybutadiene having low Mooney viscosity (less than 50) and the lanthanide-catalyzed polybutadiene having low Mooney viscosity (less than 50), the rebound characteristics and durability of the resulting golf ball are not sufficiently obtained.        
In Japanese Patent Kokai Publication No. 319148/1999, a solid golf ball comprising a core and a cover covering the core, of which the core comprises at least one rubber layer formed from a rubber composition comprising a polybutadiene mixture, unsaturated carboxylic acid or a metal salt of unsaturated carboxylic acid, organic peroxide and inorganic filler, is disclosed. The polybutadiene mixture consists of                (a) a polybutadiene containing a cis-1,4 bond of not less than 80 molar %, having a Mooney viscosity of 50 to 69 ML1+4(100° C.) and a ratio (Mw/Mn) of weight average molecular weight (Mw) to number average molecular weight (Mn) of 4.0 to 8.0, synthesized using nickel-containing catalyst and/or cobalt- containing catalyst, and        (b) a polybutadiene containing a cis-1,4 bond of not less than 40%, having a Mooney viscosity of 20 to 90 ML1+4 (100° C.), synthesized using lanthanide-containing catalyst, and a weight ratio (a)/(b) is 30/70 to 90/10. However, workability while processing is degraded and rebound characteristics of the resulting golf ball are not sufficiently obtained.        